Image copyrightAFPImage caption
They marched to the residence of Beijing's representative in Hong Kong

"For Hong Kong, this is the first time there has been such a clear violation of Hong Kong's law, a clear violation of 'one country, two systems' principle, that has taken place in such an open manner," Albert Chan, a politician with the pro-democracy People Power Party, told the BBC.

"This is why so many people have come out."

Another pro-democracy politician, Lee Cheuk-yan, said the disappearances were a "milestone for suppression".

The latest to vanish has been Lee Bo, who disappeared late last year and was last seen in Hong Kong.

Pro-democracy politicians and activists believe he was kidnapped and taken to the mainland.

Image copyrightReutersImage caption
This man decorated himself in rope that spelt out characters reading "kidnap"

Image copyrightReutersImage caption
This woman clashed with police as she tried to trample on a flag

Mr Lee raised the alarm when four of his colleagues at the tiny Causeway Bay Bookstore and related publishing house, Mighty Current, went missing in October.

One of them, publisher Gui Minhai, was last seen in Thailand. The other three were last seen in mainland China.

There has been no official comment from the Chinese government on Mr Lee's case.

But an editorial in the state-controlled Global Times newspaper said some were trying to "hype" the incident "to create estrangement between Hong Kong and the mainland".

Mr Lee is a UK passport holder and the British government said it is "deeply concerned" about his whereabouts. It says it has asked for information from the Chinese authorities.